The invention relates to a sensor for authenticity identification of signets on documents as claimed in the preamble of patent claim 1, and to a signet which interacts with the sensor and has at least one identification feature. Various embodiments of such a sensor have been disclosed. The subject matter of DE 41 17 011 A1 describes an optical sensor in which, in particular, diffuse, low-intensity radiation is intended to be detected, such as that which also occurs when checking currency bills which are provided with luminescent features.
The sensor system described there comprises a conically widening optical fiber rod and further-processing optics, in which case the radiation coming from the measurement object can be detected over a wide spatial angle using the narrow cross-section end of the fiber rod. Owing to the cross-section conversion, the radiation emerges from the fiber rod at a considerably narrower angle, which is matched to the aperture angle of the subsequent optics.
Although it is possible to detect relatively low-intensity luminescent features using this sensor, the strength of the detected luminescent features must not, however, fall below a specific threshold when they are distributed over a relatively large area. It is therefore still relatively insensitive. This is because the use of a conically formed fiber rod results in the disadvantage that detection can take place only in a region in the form of a point on the document, which fails to occur when the element to be investigated (also referred to as the identification feature) is arranged at other points on the document.
Furthermore, excitation using conventional light sources with visible light (for example incandescent lamps) leads to a relatively weak luminescence signal, which must be detected by the fiber rod and must be supplied to the evaluation optics.
The invention is therefore based on the object of developing a sensor for authenticity identification of signets on documents, such that luminescent signets on the document can be identified over a considerably larger area of the document.
In order to achieve said object, the invention is distinguished by the technical teaching of claim 1.
The major feature of the invention is that a focused beam which is emitted by a beam source is converted by focusing optics in such a manner that a scanning line, which is roughly in the form of a line, is produced on the surface of the document to be investigated and causes the identification feature, which is arranged on the document, to fluoresce, and the fluorescence signal is evaluated via detection optics by an evaluation unit.
In order to delineate the individual terms from one another, the term xe2x80x9cidentification featurexe2x80x9d is used generally as a feature which verifies the authenticity of a document, which can be applied directly to the document itself, but which can also be arranged in the region of a signet.
The term xe2x80x9csignetxe2x80x9d describes a mark or a label, a seal, a delineated area of any type or a printed region on a document which is connected (for example by being bonded on) detachably or non-detachably to the document, on which the identification feature is arranged. The later description does not define whether the identification feature is located directly on the document itself or is part of a signet applied to the document and which is connected detachably or non-detachably to the document.
The given technical teaching results in the major advantage that the production of a scanning line, which is approximately in the form of a bar, on the document to be investigated for the first time makes it possible to investigate not only areas in the form of points on the document, but an entire area in the form of a line, which is converted into a corresponding investigation area when the document is moved past the measurement window at a specific speed approximately at right angles to the longitudinal axis of the scanning line.
It is thus now for the first time possible to investigate a document which is moving past the measurement window over a large area for the presence of authenticity features, while the document is being moved past the measurement window of the sensor and, in the process, the scanning line, which is projected on the document surface, scans a relatively large area of the document. The authenticity feature or signet to be investigated can thus be arranged such that it is distributed over a large area of the document. The scanning beam is chosen to be sufficiently long that the signet is reliably arranged in the region of the scanning beam of the document.
It is preferable for the so-called up-conversion effect to be used. In this case, the excitation wavelength is longer than the wavelength emitted from the authenticity feature. Expressed in the frequency domain, this means that the excitation frequency is lower than the response frequency.
However, the invention also relates to other excitation mechanisms, such as the use of the xe2x80x9cnormalxe2x80x9d fluorescence effect, in which the excitation takes place at a specific wavelength and the fluorescent authenticity feature responds at a longer wavelength, which represents the opposite effect to said up-conversion effect.
A third embodiment relates to the fluorescence effect in which the excitation is at the same wavelength as the emission wavelength, but with the response pulse following the excitation pulse with a defined time delay.
All said effects are the subject matter of the present invention, and the protection area of the invention extends to the use of all said effects, also when combined with one another.
The further features of the invention are evident from the following summary, based on key words:
Features of the Sensor System
1. The sensor according to the invention is suitable for mounting in a (high-speed) transport apparatus, and may also be in the form of a scanner.
2. It is suitable for detection of a security feature, predominantly on flat objects.
3. A signet, comprising a color with added up-conversion pigments, is proposed by preference as the detectable security feature. If need be, these pigments can also be added directly in an applied solution, an applied paint, the adhesive or the paper.
4. The sensor is advantageously suitable for detection of a (for example printed-on) security feature with small dimensions (for example 5xc3x975 mm). If the security feature is applied by printing, the printing can be applied within relatively wide limits. The required sensor measurement range must therefore cover the entire possible printed area, although the signet printed on it may appear anywhere in this printing area, and the signet may be several times smaller than the printing area.
5. The measurement area (scanning area, transversely with respect to the transport direction) may, for example, have a size of up to 70 mm, and the small security feature is detected anywhere within this large measurement area.
6. Position-resolved detection is preferably carried out in the transport direction, but position resolution in the direction transversely with respect to the transport direction is only optional.
7. The speed in the transport direction varies from zero to 4 m/s.
8. An embodiment in the form of a two-band sensor is also preferred, in which the test object is illuminated once, and in which two different spectral bands are evaluated. In this case, the received beams are split by means of a beam splitter, are each optically filtered separately, with different pass band wavelengths, and are evaluated in separate receivers. A dichroic mirror is particularly suitable for use as a beam splitter. Alternatively, a spectral measurement could be carried out, in which the presence of one, two or more spectral components and their spectral appearance as well as their intensity would be monitored.
9. An integrated UV luminescence sensor can be used as an embodiment, in which the test object is illuminated with UV light (for example with UV LED at a wavelength of 370 nm or using a discharge lamp), and the luminescence signal is detected in a different spectral band. Specifically adapted optical filters are required for this purpose.
10. An integrated or external object detector (optical barrier) can be used as an additional feature, which indicates to the sensor when the object (signet) starts and when it ends.
11. If a synchronization input is used, which is fed with a switching signal proportional to speed, it is also possible to investigate a certain predetermined subsection of the test object on its own in the event of speed variations, or the measurement signals, position-resolved in the transport direction, can be synchronized to this synchronization signal.
12. Pigments with a rapid rise and a rapid fall time (for example typically 0.1 m/s) are advantageously used, in order to allow detection at the desired high speeds. The electronic evaluation is, of course, matched to the characteristic times of the pigments.
13. Shadowing of external light by means of mechanical apparatuses is also advantageous.
Features of the Illumination Optics (Laser Optics)
1. A laser wavelength of 980xc2x110 nm is preferable, and a wavelength of 850xc2x120 nm can also be used in another embodiment. However, LEDs or other light sources can also be used.
2. The laser line according to the invention is produced by focusing using a commercially available cylindrical lens or by focusing using a normal lens and subsequent divergence using a cylindrical lens. This results in the production of a laser line whose illumination intensity is typically at its highest in the center of the scanning line. This is due both to the imaging quality of the cylindrical lens and to the emission characteristic of the laser diode.
3. The laser line is preferably produced jointly using a normal lens and using an aspheric cylindrical lens or, alternatively, jointly using a cylindrical lens array or, in another alternative, jointly using a sinusoidal lens surface; these optics make it possible for the illumination intensity to be distributed to a certain extent homogeneously over the entire length of the laser line, or to be slightly raised at the edge (or likewise in the center), in order to compensate for the sensitivity variation of the receiver over the measurement range.
4. Alternatively, the xe2x80x9claser linexe2x80x9d can be simulated by a number of individual beams, which are relatively close to one another, in order to identify the small, marked signets.
5. When using a number of individual beams to simulate the laser line, the beams can be focused in the object plane such that they are slightly defocused, in order to achieve an optimum illumination intensity for the pigments. The xe2x80x9cluminescence efficiencyxe2x80x9d varies with the illumination intensity and is generally at an optimum at an illumination intensity which is high, but not too high. If the illumination intensity is too high, the signal level of the received light may fall again.
6. The temperature of the laser diode may be stabilized in order to stabilize its wavelength, since the fluorescence efficiency of up-conversion pigments is highly wavelength dependent.
Features of the Evaluation
1. The wavelengths which are not desired in the receiver are filtered optically. In the main embodiment in particular, the actual laser light is filtered out and is suppressed. This suppression is preferably chosen to be very intense, for example  less than 10xe2x88x927. In addition, any incident daylight is suppressed as strongly as possible. An interference bandpass filter with a pass band of 20 to 100 nm is preferably used and, additionally or if need be, short- and long-pass filters with suitable cutoff wavelengths instead of this filter.
2. In order to suppress the external light, the laser light is preferably pulsed, and the received signal is filtered using an electronic high-pass filter.
3. In order to further increase the identification confidence, the security feature must be identified during at least two, or possibly a greater number, of periods, otherwise it is rejected as being inadequate.
4. The interference immunity to external light and electronic injected interference is further improved by a conventional embodiment orxe2x80x94as will be described in the following textxe2x80x94by a specific embodiment of synchronous gain. In order to achieve synchronous gain, the evaluation is carried out via an analog circuit having sample-and-hold elements. The synchronous gain evaluates only light signals which are received in phase with the pulse repetition frequency of the transmitted laser light. Furthermore, the signal evaluation may include other details, such as sampling of the signal once 10% of the pulse duration has elapsed, and comparison of this signal with the signal once 60-95% of the pulse duration has elapsed. The signal level of the comparison signal must now be greater by a certain amount than the value sampled first of all. To do this, the chosen time windows, in this case 10% and 60-90%, must be matched to the bandpass frequency of the electronics and, in particular, to the rise and fall times of the pigments. A microprocessor is advantageously used to monitor these signals and time signals. The same principle can be used in the pulse pause for the fall in the signal. This specific embodiment of synchronous gain at the same time allows a certain amount of electronic suppression of the exciting laser light, if this has not already been completely suppressed by the optical filters.
5. Alternatively, the evaluation can be carried out using a microprocessor with an integrated or external A/D converter. The same evaluation principle can be used in this case.
6. In an alternative embodiment to the invention described above, it is proposed that a number of lenses be used, instead of a single lens, which are stacked one above the other (or in a row behind the measurement window). The light from each lens is then passed through a respective reflection cone and falls onto a respective receiver. The signals can be evaluated individually, in pairs or jointly. A total may also be formed, with this signal then being evaluated.
7. Another preferred embodiment proposes coaxial optics. In this embodiment, the transmitted and receiving beams are joined together via a beam splitter, which may also be dichroic, and leave the sensor housing together. This beam splitter is advantageously installed between the optics head and the measurement window. In this embodiment the sensitivity variation with the distance in front of the sensor is less.
A further preferred embodiment relates to the arrangement of a stack of lenses with a short focal length and a wide aperture, each having one receiving element. The various lenses each image a portion of the laser line on the associated receiving element. In order to gather a large amount of light, an imaging scale of 1:1 is preferable, together with a focal length of 10-20 mm. The distance between the lens and the object is then quite short, and this reduces the influence of scattered light.
The subject matter of the present invention results not only from the subject matter of the individual patent claims but also from the combination of the individual patent claims with one another. All the statements and features disclosed in the documentsxe2x80x94including the abstractxe2x80x94and in particular the physical embodiment illustrated in the drawings are claimed as being significant to the invention where they are novel individually or in combination in comparison to the prior art.